It is known that it is possible with a computed tomography facility, due to the rotation of the image recording facility, to record a plurality of individual images of the examination object from different directions. These individual images are then processed using an image processing facility in such a manner that a reconstruction image suitable for outputting, generally a 3D reconstruction image, can be generated and displayed. In the context of the present invention the term x-ray diagnostics facility for producing computed tomography recordings, shortened to computed tomography facility, is used to refer to both conventional computed tomography systems, which have a fixed gantry with a rotating image recording facility—whether in the form of a rotating emitter-detector unit or in the form of just a rotating emitter with a fixed detector ring—as well as x-ray facilities, for example with a C-arm, on which the radiation source and radiation detector are disposed and which can rotate about the examination object. The structure and mode of operation of computed tomography facilities of both types are sufficiently known.
To produce a computed tomography image a first recording or first scan is first carried out with a low radiation dose and low detector resolution, which can be achieved for example in the context of image processing by pixel binning, in order to obtain an overview image, which allows a first general overview or initial diagnosis. The user can then orient themselves within this overview image and select a defined region, which requires closer consideration, to which end a second recording or second scan is carried out. This defined region is now generally away from the center of the image and therefore away from the isocenter, about which the image recording facility of the computed tomography facility rotates. When the second recording or second scan is now carried out, it is done with a significantly higher dose and a higher resolution on the part of the detector, to record the image so that it is as informative as possible and it is possible to show even fine structures with sufficient clarity and contrast. It is possible to reduce the measuring field to some degree based on the maximum size in the context of the first recording. However since the image recording facility also rotates about the same isocenter during the second recording, the measuring field reduction is limited, as it must be ensured that the measuring field captures the region of interest in every instance from any recording direction. As a result the radiation load on the examination object in regions away from the defined region of actual interest is considerably high due to the necessarily relatively large measuring field during this second scan. Also the recorded image data set per individual image is considerably large due to the large measuring field.